Natural gas liquid fractionation plants low grade waste heat conversion to cooling, power and water

ABSTRACT

A method of recovering heat from a Natural Gas Liquid (NGL) fractionation plant for production of potable water. The method includes heating a buffer fluid via a heat exchanger in the NGL fractionation plant to transfer heat from the NGL fractionation plant to the buffer fluid. The method includes heating feed water with the buffer fluid discharged from the heat exchanger for production of potable water via a multi-effect-distillation (MED) system. The method may include producing potable water with heat from the buffer fluid in the MED system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure claims the benefit of priority to U.S. application Ser. No. 15/842,160 entitled, “NATURAL GAS LIQUID FRACTIONATION PLANT COOLING CAPACITY AND POTABLE WATER GENERATION USING INTEGRATED VAPOR COMPRESSION-EJECTOR CYCLE AND MODIFIED MULTI-EFFECT DISTILLATION SYSTEM,” which was filed on Dec. 14, 2017, which in turn claims the benefit of priority to U.S. Application Ser. No. 62/542,687 entitled, “Utilizing Waste Heat Recovered From Natural Gas Liquid Fractionation Plants,” which was filed on Aug. 8, 2017, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to operating industrial facilities, for example, a natural gas liquid fractionation plant or other industrial facilities that include operating plants that generate heat, for example, a natural gas liquid fractionation plant.

BACKGROUND

Natural gas liquid (NGL) processes are chemical engineering processes and other facilities used in petroleum refineries to transform natural gas into products, for example, liquefied petroleum gas (LPG), gasoline, kerosene, jet fuel, diesel oils, fuel oils, and such products. NGL facilities are large industrial complexes that involve many different processing units and auxiliary facilities, for example, utility units, storage tanks, and such auxiliary facilities. Each refinery can have its own unique arrangement and combination of refining processes determined, for example, by the refinery location, desired products, economic considerations, or such factors. The NGL processes that are implemented to transform the natural gas into the products such as those listed earlier can generate heat, which may not be reused, and byproducts, for example, greenhouse gases (GHG), which may pollute the atmosphere. It is believed that the world's environment has been negatively affected by global warming caused, in part, due to the release of GHG into the atmosphere.

SUMMARY

This specification describes technologies relating to cooling capacity generation, power generation or potable water production from waste heat in a natural gas liquid (NGL) fractionation plant.

The present disclosure includes one or more of the following units of measure with their corresponding abbreviations, as shown in Table 1:

TABLE 1 Unit of Measure Abbreviation Degrees Celsius ° C. Megawatts MW One million MM British thermal unit Btu Hour h Pounds per square inch (pressure) psi Kilogram (mass) Kg Second S Cubic meters per day m³/day Fahrenheit F.

Certain aspects of the subject matter described here can be implemented as a system. In an example implementation, the system includes a waste heat recovery heat exchanger network thermally coupled to multiple heat sources of a Natural Gas Liquid (NGL) fractionation plant. The heat exchanger network is configured to recover at least a portion of heat generated at the multiple heat sources. The system includes a first sub-system thermally coupled to the waste heat recovery heat exchanger to receive at least a first portion of heat recovered by the heat exchanger network. The first sub-system is configured to perform one or more operations using at least the first portion of heat recovered by the heat exchanger network.

In an aspect combinable with the example implementation, the system includes a second sub-system thermally coupled to the waste heat recovery heat exchanger to receive at least a second portion of heat recovered by the heat exchanger network. The second sub-system is separate and distinct from the first sub-system, and is configured to perform one or more operations using at least the second portion of heat recovered by the heat exchanger network.

In another aspect combinable with any of the previous aspects, the system includes a control system connected to the heat exchanger network and the first sub-system, or the heat exchanger network and the second sub-system, or the heat exchanger network, the first sub-system and the second sub-system. The control system is configured to flow fluids between the NGL fractionation plant, the heat exchanger network one or both of the first sub-system or the second sub-system.

In another aspect combinable with any of the previous aspects, the fluids include one or more of a NGL fractionation plant stream or a buffer fluid.

In another aspect combinable with any of the previous aspects, the multiple heat sources include first multiple sub-units of the NGL fractionation plant including a propane dehydration section, a de-propanizer section, a butane de-hydrator section, and a de-butanizer section, a second multiple sub-units of the NGL fractionation plant including a de-pentanizer section, an Amine-Di-Iso-Propanol (ADIP) regeneration section, a natural gas de-colorizing section, a propane vapor recovery section and a propane product refrigeration section, and third multiple sub-units of the NGL fractionation a propane product sub-cooling section, a butane product refrigeration section, an ethane production section and a Reid Vapor Pressure (RVP) control section.

In another aspect combinable with any of the previous aspects, the heat exchanger network includes multiple heat exchangers.

In another aspect combinable with any of the previous aspects, the multiple heat exchangers include a first subset comprising one or more of the multiple heat exchangers thermally coupled to the first plurality of sub-units of the NGL fractionation plant.

In another aspect combinable with any of the previous aspects, the first subset includes a first heat exchanger thermally coupled to the propane dehydration section, and configured to heat a first buffer stream using heat carried by a propane de-hydration outlet stream from the propane de-hydration section. The first subset includes a second heat exchanger thermally coupled to the de-propanizer section, and configured to heat a second buffer stream using heat carried by a de-propanizer overhead outlet stream from the de-propanizer section. The first subset includes a third heat exchanger thermally coupled to the butane de-hydrator section, and configured to heat a third buffer stream using heat carried by a butane de-hydrator outlet stream. The first subset includes a fourth heat exchanger thermally coupled to the de-butanizer section, and configured to heat a fourth buffer stream using heat carried by a de-butanizer overhead outlet stream from the de-butanizer section. The first subset includes a fifth heat exchanger thermally coupled to the de-butanizer section, and configured to heat a fifth buffer stream using heat carried by a de-butanizer bottoms outlet stream from the de-butanizer section.

In another aspect combinable with any of the previous aspects, the multiple heat exchangers include a second subset including one or more of the multiple heat exchangers thermally coupled to the second plurality of sub-units of the NGL fractionation plant.

In another aspect combinable with any of the previous aspects, the second subset includes a sixth heat exchanger thermally coupled to the de-pentanizer section, and configured to heat a sixth buffer stream using heat carried by a de-pentanizer overhead outlet stream from the de-pentanizer section. The second subset includes a seventh heat exchanger thermally coupled to the ADIP regeneration section, and configured to heat a seventh buffer stream using heat carried by an ADIP regeneration section overhead outlet stream. The second subset includes an eighth heat exchanger thermally coupled to the ADIP regeneration section, and configured to heat an eighth buffer stream using heat carried by an ADIP regeneration section bottoms outlet stream. The second subset includes a ninth heat exchanger thermally coupled to the natural gas de-colorizing section, and configured to heat a ninth buffer stream using heat carried by a natural gas de-colorizing section pre-flash drum overhead outlet stream. The second subset includes a tenth heat exchanger thermally coupled to the natural gas de-colorizing section, and configured to heat a tenth buffer stream using heat carried by a natural gas de-colorizer overhead outlet stream. The second subset includes an eleventh heat exchanger thermally coupled to the propane vapor recovery section, and configured to heat an eleventh buffer stream using heat carried by a propane vapor recovery compressor outlet stream. The third subset includes a twelfth heat exchanger thermally coupled to the propane product refrigeration section, and configured to heat a twelfth buffer stream using heat carried by a propane refrigeration compressor outlet stream from the propane product refrigeration section.

In another aspect combinable with any of the previous aspects, the multiple heat exchangers include a third subset including one or more of the multiple heat exchangers thermally coupled to the third plurality of sub-units of the NGL fractionation plant.

In another aspect combinable with any of the previous aspects, the third subset includes a thirteenth heat exchanger thermally coupled to the propane product sub-cooling, and configured to heat a thirteenth buffer stream using heat carried by a propane main compressor outlet stream from the propane product sub-cooling section. The third subset includes a fourteenth heat exchanger thermally coupled to the butane product refrigeration section, and configured to heat a fourteenth buffer stream using heat carried by a butane refrigeration compressor outlet stream from the butane product refrigeration section. The third subset includes a fifteenth heat exchanger thermally coupled to the ethane production section, and configured to heat a fifteenth buffer stream using heat carried by an ethane dryer outlet stream. The third subset includes a sixteenth heat exchanger thermally coupled to the RVP control section, and configured to heat a sixteenth buffer stream using heat carried by a RVP control column overhead outlet stream.

In another aspect combinable with any of the previous aspects, the system includes a storage tank configured to store the buffer streams. The control system is configured to flow the buffer streams from the storage tank to the heat exchanger network.

In another aspect combinable with any of the previous aspects, the buffer stream includes pressurized water.

In another aspect combinable with any of the previous aspects, the first sub-system includes a modified multi-effect-distillation (MED) system configured to produce potable water using at least the portion of heat recovered by the heat exchanger network. The MED system includes multiple trains. Each train is configured to receive the heated buffer fluid from the heat exchanger network and to produce potable water using the heat carried by the heated buffer fluid.

In another aspect combinable with any of the previous aspects, the MED system includes three trains. A first train includes six effects, a second train includes four effects and a third train includes two effects.

In another aspect combinable with any of the previous aspects, the second sub-system includes a cooling sub-system configured to generate cooling capacity to cool at least a portion of the NGL fractionation plant.

In another aspect combinable with any of the previous aspects, the cooling sub-system includes a mono-refrigerant dual vapor compressor-ejector cycle.

In another aspect combinable with any of the previous aspects, the compressor-ejector cycle includes a first propane stream that is vaporized to generate the cooling capacity.

In another aspect combinable with any of the previous aspects, the third subset includes a third subset of heat exchangers including a seventeenth heat exchanger thermally coupled to the de-ethanizer section, and configured to heat the first propane stream using heat carried by a de-ethanizer refrigeration compressor outlet stream from the de-ethanizer section.

In another aspect combinable with any of the previous aspects, the MED system can include a first phase comprising three trains. A first train in the first phase can include six effects, a second train in the first phase can include four effects and a third train in the first phase can include two effects.

In another aspect combinable with any of the previous aspects, the MED system can include a second phase including three trains. The second phase can be connected in parallel with the first phase. A fourth train in the second phase can include five effects, a fifth train in the second phase can include four effects and a sixth train in the second phase can include two effects.

In another aspect combinable with any of the previous aspects, the MED system can include a third phase including one train, the third phase connected in parallel with the second phase.

The details of one or more implementations of the subject matter described in this specification are set forth in the accompanying drawings and the detailed description. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of an example of a low grade waste heat recovery system.

FIG. 1B is a schematic diagram of a propane de-hydration section waste heat recovery system in a NGL fractionation plant.

FIG. 1C is a schematic diagram of a de-propanizer section waste heat recovery system in a NGL fractionation plant.

FIG. 1D is a schematic diagram of a butane de-hydrator section waste heat recovery system in a NGL fractionation plant.

FIG. 1E is a schematic diagram of a de-butanizer section waste heat recovery system in a NGL fractionation plant.

FIG. 1F is a schematic diagram of a de-pentanizer section waste heat recovery system in a NGL fractionation plant.

FIG. 1G is a schematic diagram of an ADIP regeneration section waste heat recovery system in a NGL fractionation plant.

FIG. 1H is a schematic diagram of a natural gasoline de-colorizing section waste heat recovery system in a NGL fractionation plant.

FIG. 1I is a schematic diagram of a propane tank vapor recovery section waste heat recovery system in a NGL fractionation plant.

FIG. 1J is a schematic diagram of a propane product refrigeration section waste heat recovery system in a NGL fractionation plant.

FIG. 1K is a schematic diagram of a propane product sub-cooling section waste heat recovery system in a NGL fractionation plant.

FIG. 1L is a schematic diagram of a butane product refrigeration waste heat recovery system in a NGL fractionation plant.

FIG. 1M is a schematic diagram of an ethane production section waste heat recovery system in a NGL fractionation plant.

FIG. 1N is a schematic diagram of a natural gasoline vapor pressure control section waste heat recovery system in a NGL fractionation plant.

FIG. 1O is a schematic diagram of an integrated, customized, mono-refrigerant dual vapor compressor-ejector cycle.

FIG. 1P is a schematic diagram representing an example of a modified MED system configured to produce potable water using pressurized water heated by a heat exchanger network.

DETAILED DESCRIPTION

NGL Plant

Gas processing plants can purify raw natural gas or crude oil production associated gases (or both) by removing common contaminants such as water, carbon dioxide and hydrogen sulfide. Some of the substances which contaminate natural gas have economic value and can be processed or sold or both. Upon the separation of methane gas, which is useful as sales gas for houses and power generation, the remaining hydrocarbon mixture in liquid phase is called natural gas liquids (NGL). The NGL is fractionated in a separate plant or sometimes in the same gas processing plant into ethane, propane and heavier hydrocarbons for several versatile uses in chemical and petrochemical as well as transportation industries. The NGL fractionation plant uses the following processes or sections: fractionation, product treating, and natural gasoline processing. The fractionation processes or sections can include heat sources (also commonly referred to as streams) including, but not limited to, a propane condenser, a propane refrigerant condenser, a naphtha cooler, a de-pentanizer condenser, an amine-di-iso-propanol (ADIP) cooler, a regenerator overhead (OVHD) condenser, a Reid vapor pressure (RVP) column condenser, a de-propanizer condenser, a de-butanizer condenser, or combinations thereof. The product treating processes or sections can include the following non-limiting heat sources: a propane dehydrator condenser, a butane dehydrator condenser, a propane condenser, an air-cooled condenser, a regeneration gas cooler, and a butane condenser, or combinations thereof. The natural gasoline processing processes or sections can include, but are not limited to, a natural gasoline (NG) flash vapor condenser, a NG de-colorizer condenser, or combinations thereof.

Fractionation Section

Fractionation is the process of separating the different components of natural gas. Separation is possible because each component has a different boiling point. At temperatures less than than the boiling point of a particular component, that component condenses to a liquid. It is also possible to increase the boiling point of a component by increasing the pressure. By using columns operating at different pressures and temperatures, the NGL fractionation plant is capable of separating ethane, propane, butane, pentane, or combinations thereof (with or without heavier associated hydrocarbons) from NGL fractionation feeds. De-ethanizing separates ethane from C2+ NGL, where C2 refers to a molecule containing two carbon atoms (ethane), and where C2+ refers to a mixture containing molecules having two or more carbon atoms, for example, a NGL containing C2, C3, C4, C5 can be abbreviated as “C2+ NGL”. De-propanizing and de-butanizing separate propane and butane, respectively, from C3+ NGL and C4+ NGL, respectively. Because the boiling points of heavier natural gases are closer to each other, such gases can be harder to separate compared to lighter natural gases. Also, a rate of separation of heavier components is less than that of comparatively lighter components. In some instances, the NGL fractionation plant can implement, for example, about 45 distillation trays in the de-ethanizer, about 50 trays in the de-propanizer, and about 55 trays in the de-butanizer.

The fractionation section can receive a feed gas containing C2+ NGL from gas plants, which are upstream plants that condition and sweeten the feed gas, and produce a sales gas, such as a C1/C2 mixture, where C1 is about 90%, as a final product. The C2+ NGL from gas plants can be further processed in the NGL fractionation plant for C2+ recovery. From feed metering or surge unit metering (or both), feed flows to the three fractionation modules, namely, the de-ethanizing module, the de-propanizing module and the de-butanizing module, each of which is described later.

De-Ethanizer Module (or De-Ethanizer Column)

The C2+ NGL is pre-heated before entering the de-ethanizer column for fractionation. The separated ethane leaves the column as overhead gas. The ethane gas is condensed by a closed-loop propane refrigeration system. After being cooled and condensed, the ethane is a mixture of gas and liquid. The liquid ethane is separated and pumped back to the top of the column as reflux. The ethane gas is warmed in an economizer and then sent to users. The bottoms product from the de-ethanizer reboiler is C3+ NGL, which is sent to the de-propanizer module.

De-Propanizer Module (or De-Propanizer Column)

From the de-ethanizer module, C3+ NGL enters the de-propanizer module for fractionation. The separated propane leaves the column as overhead gas. The gas is condensed using coolers. The propane condensate is collected in a reflux drum. Some of the liquid propane is pumped back to the column as reflux. The rest of the propane is either treated or sent to users as untreated product. The bottoms product from the depropanizer reboiler, C4+ is then sent to the de-butanizer module

De-Butanizer Module (or De-Butanizer Column)

C4+ enters the de-butanizer module for fractionation. The separated butane leaves the column as overhead gas. The gas is condensed using coolers. The butane condensate is collected in a reflux drum. Some of the liquid butane is pumped back to the column as reflux. The rest of the butane is either treated or sent to users as untreated product. The bottoms product from the debutanizer reboiler, C5+ natural gas (NG) goes on to a RVP control section (which may also be referred to as a rerun unit), which will be discussed in greater detail in a later section.

Product Treating Section

While ethane requires no further treatment, propane and butane products are normally treated to remove hydrogen sulfide (H₂S), carbonyl sulfide (COS), and mercaptan sulfur (RSH). Then, the products are dried to remove any water. All exported product is treated, while untreated products can go to other industries. As described later, propane receives ADIP treating, MEROX™ (Honeywell UOP; Des Plaines, Ill.) treating, and dehydration. Butane receives MEROX treating, and dehydration.

ADIP Treating Section

ADIP is a solution of di-isopropanol amine and water. ADIP treating extracts H₂S and COS from propane. The ADIP solution, through contact with the sour propane, absorbs the H₂S and COS. The ADIP solution first contacts the sour propane in an extractor. In the extractor, the ADIP absorbs most of the H₂S and some of the COS. The propane then passes through a mixer/settler train where the propane contacts with ADIP solution to extract more H₂S and COS. This partially sweetened propane is cooled and then washed with water to recover the ADIP entrained with the propane. The propane is then sent to MEROX treating, which is described later. The rich ADIP that has absorbed the H₂S and COS leaves the bottom of the extractor and is regenerated into lean ADIP for reuse. The regenerator column has a temperature and pressure that are suitable for acid gas removal. When the rich ADIP enters the regenerator, the entrained acid gases are stripped. As the acid gases leaves the regenerator as overhead, any free water is removed to prevent acid formation. The acid gases are then sent to flare. The lean ADIP leaves the extractor bottom and is cooled and filtered. Lean ADIP returns to the last mixer/settler and flows back through the system in the counter-current direction of the propane to improve contact between the propane and ADIP, which improves H₂S and COS extraction.

C3/C4 MEROX Treating Section

MEROX treating removes mercaptan sulfur from C3/C4 product. Mercaptans are removed using a solution of sodium hydroxide (NaOH), also known by the commercial name caustic soda (hereinafter referred to as “caustic”) and MEROX. The MEROX catalyst facilitates the oxidation of mercaptans to disulfides. The oxidation takes place in an alkaline environment, which is provided by using the caustic solution. MEROX treating for C3 and C4 is similar. Both products are prewashed with caustic to remove any remaining traces of H₂S, COS, and CO₂. This prevents damage to the caustic that is used in MEROX treating. After prewashing, product flows to an extractor, where a caustic solution with MEROX catalyst contacts with the product. The caustic/catalyst solution converts the mercaptans into mercaptides. The sweetened product, which is lean on acid gases, leaves the extractor as overhead and any remaining caustic is separated. Caustic leaves the bottom of both product extractors rich with mercaptides. The rich caustic is regenerated into lean caustic for reuse. The C3/C4 extraction sections share a common caustic regeneration section, namely, an oxidizer. Before entering the bottom of the oxidizer, the rich caustic is injected with MEROX catalyst to maintain proper catalyst concentration, heated, and mixed with process air. In the oxidizer, the mercaptides are oxidized into disulfides. The mixture of disulfides, caustic, and air leave the oxidizer as overhead. The air, disulfide gases, and disulfide oil are separated from the regenerated caustic. The regenerated caustic is pumped to the C3/C4 extractor. Regenerated caustic with any residual disulfides is washed with NG in the NG wash settler.

C3/C4 Dehydration Section

Propane or butane products (or both) contain water when they leave MEROX treating. Dehydration removes moisture in such products through adsorption before the products flow to refrigeration and storage. The dehydration processes for C3 and C4 are similar. Both C3/C4 dehydration sections have two de-hydrators containing molecular sieve desiccant beds. One de-hydrator is in service while the other undergoes regeneration. Regeneration consists of heating the sieve beds to remove moisture, then cooling the beds before reuse. During drying, product flows up and through the molecular sieve bed, which adsorbs (that is, binds to its surface) moisture. From the top of the de-hydrator, dry C3/C4 products flow to refrigeration.

Natural Gasoline (NG) Processing Section

NG processing includes RVP control, de-colorizing and de-pentanizing sections.

RVP Control Section

A Reid vapor pressure (RVP) control section (or rerun unit) is a fractionator column that receives the C5+ NG from the debutanizer bottom. The RVP control section collects a pentane product. The RVP control section can be used to adjust the RVP of the pentane product at a rerun fractionator overhead before the pentane product is sent to a pentane storage tank. RVP is a measure of the ability of a hydrocarbon to vaporize. RVP (sometimes called volatility) is an important specification in gasoline blending. The RVP control section stabilizes the RVP of NG by removing small amounts of pentane. Depending on operational requirements, the RVP control section can be totally or partially bypassed. NG from the debutanizer bottoms goes to the RVP column where a controlled amount of pentane is stripped and leaves the column as overhead gas. As in NGL fractionation, the overhead gas is condensed with coolers, and some of the condensate is pumped back to the column as reflux. The remaining pentane is cooled and sent to storage. If the RVP column bottoms product (NG) meets color specifications, it is sent to storage. If not, it is sent to decolorizing.

De-Colorizing Section

The de-colorizing section removes color bodies from NG. Color bodies are traces of heavy ends found in the de-butanizer bottoms product. Other impurities such as corrosion products from the pipeline may also be present. These must be removed for NG to meet the color specification. De-colorizer feed can be RVP column bottoms product or de-butanizer bottoms product, or a combination of both. Additional natural gasoline can also be supplied from other facilities to maintain a hexane plus (C6+) product supply. If de-colorizing is needed, NG first passes through a pre-flash-drum. A large portion of the lighter NG components vaporizes and leaves the drum as overhead. The heavier NG components remain along with the color bodies and are fed to the de-colorizer column, where the remaining color bodies are separated. The NG leaves the de-colorizer as overhead gas and is condensed and collected in the NG product drum, with some pumped back to the column as reflux. Overhead from the column and flash drum are joined and pumped to either the de-pentanizer (described later) or cooled and sent to storage in the feed product surge unit. The color bodies leave the de-colorizer as bottoms product and are pumped to the feed and surge unit to be injected into a crude line.

De-Pentanizing Section

De-pentanizing uses a fractionation column to produce a pentane overhead product and a C6+ bottoms product. Both the pentane product and the C6+ bottoms product are separately fed to storage or downstream the petrochemical plants. The feed to the de-pentanizer is the NG product stream from the de-colorizing section. Feed can be increased or decreased based on the demand for C6+ bottoms product. If the NGL fractionation plant NG production cannot meet demand, NG can be imported from oil refineries. The de-colorized NG is preheated before entering the de-pentanizer. The separated pentane leaves the column as overhead gas. The overhead condensers cool the overhead stream, and some is pumped back to the column as reflux. The remaining pentane is cooled and sent to storage. Light NG in the bottoms is vaporized and returned to heat the de-pentanizer. The remaining bottoms product is cooled and sent to storage as C6+.

Table 2 lists duty per train of major waste heat streams in an example of an NGL fractionation plant.

TABLE 2 Duty/train Stream Name (MMBtu/h) Propane refrigerant condenser 94 Propane de-hydration condenser 22 Butane de-hydrator condenser 9 Naphtha cooler 11 De-pentanizer condenser 100 ADIP cooler 73 Regenerator OVHD condenser 18 NG flash vapor condenser 107 NG de-colorizer condenser 53 Natural gasoline (cooling) process 29 propane condenser Fractionation propane condenser 81 Air cooled condenser 16 Regeneration gas cooler 22 RVP column condenser 36 Butane condenser 49 De-propanizer condenser 194 De-butanizer condenser 115

In Table 2, “Duty/train” represents each stream's thermal duty in millions Btu per hour (MMBtu/h) per processing train. A typical NGL fractionation plant includes three to four processing trains.

The systems described in this disclosure can be integrated with a NGL fractionation plant to make the fractionation plant more energy efficient or less polluting or both. In particular, the energy conversion system can be implemented to recover low grade waste heat from the NGL fractionation plant. Low grade waste heat is characterized by a temperature difference between a source and sink of the low grade heat steam being between 65° C. and 232° C. (150° F. and 450° F.). The NGL fractionation plant is an attractive option for integration with energy conversion systems due to a large amount of low grade waste heat generated by the plant and an absence of a need for deep cooling. Deep cooling refers to a temperature that is less than ambient that uses a refrigeration cycle to maintain.

The low grade waste heat from an NGL fractionation plant can be used for commodities such as carbon-free power generation, cooling capacity generation, potable water production from sea water, or combinations thereof. Low grade waste heat is characterized by a temperature ranging between 65° C. and 232° C. (150° F. to 450° F.). The waste heat can be used for the mono-generation, co-generation, or tri-generation of one or more or all of the commodities mentioned earlier. Low grade waste heat from the NGL fractionation plant can be used to provide in-plant sub-ambient cooling, thus reducing the consumption of power or fuel (or both) of the plant. Low grade waste heat from the NGL fractionation plant can be used to provide ambient air conditioning or cooling in the industrial community or in a nearby non-industrial community, thus helping the community to consume energy from alternative sources. In addition, the low grade waste heat can be used to desalinate water and produce potable water to the plant and adjacent community. An NGL fractionation plant is selected for low grade waste heat recovery because of a quantity of low grade waste heat available from the NGL fractionation plant as well as a cooling requirement of the plant to ambient temperature cooling (instead of deep cooling).

The energy conversion systems described in this disclosure can be integrated into an existing NGL fractionation plant as a retrofit or can be part of a newly constructed NGL fractionation plant. A retrofit to an existing NGL fractionation plant allows the carbon-free power generation, and fuel savings advantages offered by the energy conversion systems described here to be accessible with a reduced capital investment. For example, the energy conversion systems described here can produce one or more or all of substantially between 35 MW and 40 MW (for example, 37 MW) of carbon-free power, substantially between 100,000 and 150,000 m³/day (for example, 120,000 m³/day) of desalinated water, and substantially between 350 MM BTU/h and 400 MM BTU/h (for example, 388 MM BTU/h) of cooling capacity for in-plant or community utilization or both.

As described later, the systems for waste heat recovery and re-use from the NGL fractionation plant can include modified multi-effect distillation (MED) systems, customized Organic Rankine Cycle (ORC) systems, unique ammonia-water mixture Kalina cycle systems, customized modified Goswami cycle systems, mono-refrigerant specific vapor compression-ejector-expander triple cycle systems, or combinations of one or more of them. Details of each disclosure are described in the following paragraphs.

Heat Exchangers

In the configurations described in this disclosure, heat exchangers are used to transfer heat from one medium (for example, a stream flowing through a plant in a NGL fractionation plant, a buffer fluid or such medium) to another medium (for example, a buffer fluid or different stream flowing through a plant in the NGL fractionation plant). Heat exchangers are devices which transfer (exchange) heat typically from a hotter fluid stream to a relatively less hotter fluid stream. Heat exchangers can be used in heating and cooling applications, for example, in refrigerators, air conditions or such cooling applications. Heat exchangers can be distinguished from one another based on the direction in which fluids flow. For example, heat exchangers can be parallel-flow, cross-flow or counter-current. In parallel-flow heat exchangers, both fluid involved move in the same direction, entering and exiting the heat exchanger side-by-side. In cross-flow heat exchangers, the fluid path runs perpendicular to one another. In counter-current heat exchangers, the fluid paths flow in opposite directions, with one fluid exiting whether the other fluid enters. Counter-current heat exchangers are sometimes more effective than the other types of heat exchangers.

In addition to classifying heat exchangers based on fluid direction, heat exchangers can also be classified based on their construction. Some heat exchangers are constructed of multiple tubes. Some heat exchangers include plates with room for fluid to flow in between. Some heat exchangers enable heat exchange from liquid to liquid, while some heat exchangers enable heat exchange using other media.

Heat exchangers in a NGL fractionation plant are often shell and tube type heat exchangers which include multiple tubes through which fluid flows. The tubes are divided into two sets—the first set contains the fluid to be heated or cooled; the second set contains the fluid responsible for triggering the heat exchange, in other words, the fluid that either removes heat from the first set of tubes by absorbing and transmitting the heat away or warms the first set by transmitting its own heat to the fluid inside. When designing this type of exchanger, care must be taken in determining the correct tube wall thickness as well as tube diameter, to allow optimum heat exchange. In terms of flow, shell and tube heat exchangers can assume any of three flow path patterns.

Heat exchangers in NGL facilities can also be plate and frame type heat exchangers. Plate heat exchangers include thin plates joined together with a small amount of space in between, often maintained by a rubber gasket. The surface area is large, and the corners of each rectangular plate feature an opening through which fluid can flow between plates, extracting heat from the plates as it flows. The fluid channels themselves alternate hot and cold liquids, meaning that the heat exchangers can effectively cool as well as heat fluid. Because plate heat exchangers have large surface area, they can sometimes be more effective than shell and tube heat exchangers.

Other types of heat exchangers can include regenerative heat exchangers and adiabatic wheel heat exchangers. In a regenerative heat exchanger, the same fluid is passed along both sides of the exchanger, which can be either a plate heat exchanger or a shell and tube heat exchanger. Because the fluid can get very hot, the exiting fluid is used to warm the incoming fluid, maintaining a near constant temperature. Energy is saved in a regenerative heat exchanger because the process is cyclical, with almost all relative heat being transferred from the exiting fluid to the incoming fluid. To maintain a constant temperature, a small quantity of extra energy is needed to raise and lower the overall fluid temperature. In the adiabatic wheel heat exchanger, an intermediate liquid is used to store heat, which is then transferred to the opposite side of the heat exchanger. An adiabatic wheel consists of a large wheel with threads that rotate through the liquids—both hot and cold—to extract or transfer heat. The heat exchangers described in this disclosure can include any one of the heat exchangers described earlier, other heat exchangers, or combinations of them.

Each heat exchanger in each configuration can be associated with a respective thermal duty (or heat duty). The thermal duty of a heat exchanger can be defined as an amount of heat that can be transferred by the heat exchanger from the hot stream to the cold stream. The amount of heat can be calculated from the conditions and thermal properties of both the hot and cold streams. From the hot stream point of view, the thermal duty of the heat exchanger is the product of the hot stream flow rate, the hot stream specific heat, and a difference in temperature between the hot stream inlet temperature to the heat exchanger and the hot stream outlet temperature from the heat exchanger. From the cold stream point of view, the thermal duty of the heat exchanger is the product of the cold stream flow rate, the cold stream specific heat and a difference in temperature between the cold stream outlet from the heat exchanger and the cold stream inlet temperature from the heat exchanger. In several applications, the two quantities can be considered equal assuming no heat loss to the environment for these units, particularly, where the units are well insulated. The thermal duty of a heat exchanger can be measured in watts (W), megawatts (MW), millions of British Thermal Units per hour (Btu/hr), or millions of kilocalories per hour (Kcal/h). In the configurations described here, the thermal duties of the heat exchangers are provided as being “about X MW,” where “X” represents a numerical thermal duty value. The numerical thermal duty value is not absolute. That is, the actual thermal duty of a heat exchanger can be approximately equal to X, greater than X or less than X.

Flow Control System

In each of the configurations described later, process streams (also called “streams”) are flowed within each plant in a NGL fractionation plant and between plants in the NGL fractionation plant. The process streams can be flowed using one or more flow control systems implemented throughout the NGL fractionation plant. A flow control system can include one or more flow pumps to pump the process streams, one or more flow pipes through which the process streams are flowed and one or more valves to regulate the flow of streams through the pipes.

In some implementations, a flow control system can be operated manually. For example, an operator can set a flow rate for each pump and set valve open or close positions to regulate the flow of the process streams through the pipes in the flow control system. Once the operator has set the flow rates and the valve open or close positions for all flow control systems distributed across the NGL fractionation plant, the flow control system can flow the streams within a plant or between plants under constant flow conditions, for example, constant volumetric rate or such flow conditions. To change the flow conditions, the operator can manually operate the flow control system, for example, by changing the pump flow rate or the valve open or close position.

In some implementations, a flow control system can be operated automatically. For example, the flow control system can be connected to a computer system to operate the flow control system. The computer system can include a computer-readable medium storing instructions (such as flow control instructions and other instructions) executable by one or more processors to perform operations (such as flow control operations). An operator can set the flow rates and the valve open or close positions for all flow control systems distributed across the NGL fractionation plant using the computer system. In such implementations, the operator can manually change the flow conditions by providing inputs through the computer system. Also, in such implementations, the computer system can automatically (that is, without manual intervention) control one or more of the flow control systems, for example, using feedback systems implemented in one or more plants and connected to the computer system. For example, a sensor (such as a pressure sensor, temperature sensor or other sensor) can be connected to a pipe through which a process stream flows. The sensor can monitor and provide a flow condition (such as a pressure, temperature, or other flow condition) of the process stream to the computer system. In response to the flow condition exceeding a threshold (such as a threshold pressure value, a threshold temperature value, or other threshold value), the computer system can automatically perform operations. For example, if the pressure or temperature in the pipe exceeds the threshold pressure value or the threshold temperature value, respectively, the computer system can provide a signal to the pump to decrease a flow rate, a signal to open a valve to relieve the pressure, a signal to shut down process stream flow, or other signals.

In some implementations, the techniques described here can be implemented using a waste heat recovery network that includes 17 heat exchanger units distributed in specific areas of the NGL fractionation plant. As described later, low grade waste heat can be recovered from several processing units at which the heat exchanger units are installed using two buffer streams, for example, pressurized water, pressurized liquid propane, oil or such buffer streams. The pressurized water can flow from a dedicated storage tank at a temperature of between 115° F. and 125° F. (for example, a temperature of 120° F.) towards specific units in the NGL fractionation plant to recover a specific amount of thermal energy.

The techniques can be implemented to increase the temperature of a first pressurized water stream from about 120° F. to between 150° F. and 160° F. (for example, about 158° F.) by absorbing thermal energy between 2400 MM Btu/h and 2600 MM Btu/h (for example, 2500 MM Btu/h). The pressurized water stream at about 158° F. is used to drive a modified multi-effect-distillation (MED) system to produce desalinated water from brackish water or sea water stream at the rate of about 114,000 m³/day. The temperature of the pressurized water stream is reduced back to about 120° F. The pressurized water then flows back to the storage tank and the processes are repeated.

The techniques can also be implemented to cause the second pressurized liquid propane stream to absorb between 350 MM Btu/h and 450 MM Btu/h (for example, about 400 MM plant Btu/h) from the NGL fractionation plant. The absorbed thermal energy generates between 90 MM Btu/h and 100 MM Btu/h (for example, 98 MM Btu/h) of sub-ambient cooling capacity, which is used in the plant mechanical compression based refrigeration system for the system propane refrigerant sub-cooling using vapor-ejector to decrease the plant refrigeration system power consumption by between 10 MW and 20 MW (for example, about 14 MW). The second buffer stream of propane liquid at high pressure is vaporized in a heat exchanger (described and shown later), and used as a motive vapor in the ejector, then condensed in a water cooler to be reused back in the cycle.

FIG. 1A is a schematic diagram of an example of a low grade waste heat recovery system. The schematic includes a storage tank 605 to store buffer fluid, for example, pressurized water, oil, or other buffer fluid. The buffer fluid is flowed to a heat exchanger network 607 which, in some implementations, can include seventeen heat exchangers (for example, heat exchangers 6 a, 6 b, 6 c, 6 d, 6 e, 6 f, 6 g, 6 h, 6 i, 6 j, 6 k, 6 l, 6 m, 6 n, 6 o, 6 p and 6 q), which are described in detail later. The buffer fluid is flowed through the heat exchanger network 607 and heated by streams in the NGL fractionation plant (described later). The heated buffer fluid is flowed to a MED system 609 that can generate potable water as described later. The temperature of the buffer fluid decreases as it exits the MED system 609 and flows back to the storage tank 605 to be flowed again through the heat exchanger network 607. The system also includes a cooling sub-system 611 (described later) that includes a second buffer stream, for example, pressurized propane that can be heated using one or more of the heat exchangers in the heat exchanger network 607 to reduce the cooling requirement of the cooling sub-system. The second buffer fluid sub-cools the compressor feed stream and thus reduces the compressor feed vapor density, which, in turn, reduces the power consumption of the refrigeration compressor. In some implementations, the techniques described here can be implemented with one of the MED system 609 or the cooling sub-system 611 and without the other of the MED system 609 or the cooling sub-system 611.

FIG. 1B is a schematic diagram of a propane de-hydration section waste heat recovery system in a NGL fractionation plant. A first heat exchanger 6 a is located in the propane de-hydration section of the NGL fractionation plant. In some implementations, the buffer fluid in the storage tank 605 is pressurized water at a temperature of between 115° F. and 125° F. (for example, about 120° F.). The pressurized water stream flows from the storage tank 605 to the first heat exchanger 6 a to cool down the propane de-hydration outlet stream. In turn, the temperature of the pressurized water stream increases to between 390° F. and 400° F. (for example, about 395° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the first heat exchanger 6 a is between 95 MM Btu/h and 100 MM Btu/h (for example, about 96 MM Btu/h).

FIG. 1C is a schematic diagram of a de-propanizer section waste heat recovery system in the NGL fractionation plant. A second heat exchanger 6 b is located in the de-propanizer section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the second heat exchanger 6 b to cool down the de-propanizer overhead outlet stream. In turn, the temperature of the pressurized water stream increases to between 130° F. and 140° F. (for example, about 134° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the second heat exchanger 6 b is between 950 MM Btu/h and 960 MM Btu/h (for example, about 951 MM Btu/h).

FIG. 1D is a schematic diagram of a butane de-hydrator section waste heat recovery system in the NGL fractionation plant. A third heat exchanger 6 c is located in the butane de-hydrator section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the third heat exchanger 6 c to cool down the butane de-hydrator outlet stream. In turn, the temperature of the pressurized water stream increases to between 390° F. and 400° F. (for example, about 395° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the third heat exchanger 6 c is between 40 MM Btu/h and 50 MM Btu/h (for example, about 47 MM Btu/h).

FIG. 1E is a schematic diagram of a de-butanizer section waste heat recovery system in the NGL fractionation plant. A fourth heat exchanger 6 d is located in the de-butanizer section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the fourth heat exchanger 6 d to cool down the de-butanizer overhead outlet stream. In turn, the temperature of the pressurized water stream increases to between 150° F. and 160° F. (for example, about 152° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the fourth heat exchanger 6 d is between 580 MM Btu/h and 590 MM Btu/h (for example, about 587 MM Btu/h).

Also as shown in FIG. 1E, a fifth heat exchanger 6 e is located in the de-butanizer section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the fifth heat exchanger 6 e to cool down the de-butanizer bottoms outlet stream. In turn, the temperature of the pressurized water stream increases to between 255° F. and 265° F. (for example, about 261° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the fifth heat exchanger 6 e is between 50 MM Btu/h and 60 MM Btu/h (for example, about 56 MM Btu/h).

FIG. 1F is a schematic diagram of a de-pentanizer section waste heat recovery system in the NGL fractionation plant. A sixth heat exchanger 6 f is located in the de-pentanizer section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the sixth heat exchanger 6 f to cool down the de-pentanizer overhead outlet stream. In turn, the temperature of the pressurized water stream increases to between 160° F. and 170° F. (for example, about 165° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the sixth heat exchanger 6 f is between 95 MM Btu/h and 105 MM Btu/h (for example, about 100 MM Btu/h).

FIG. 1G is a schematic diagram of an ADIP regeneration section waste heat recovery system in the NGL fractionation plant. A seventh heat exchanger 6 g is located in the ADIP regeneration section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the seventh heat exchanger 6 g to cool down the ADIP regeneration section overhead outlet stream. In turn, the temperature of the pressurized water stream increases to between 220° F. and 230° F. (for example, about 227° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the seventh heat exchanger 6 g is between 10 MM Btu/h and 20 MM Btu/h (for example, about 18 MM Btu/h).

Also as shown in FIG. 1G, an eighth heat exchanger 6 h is located in the ADIP regeneration section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the eighth heat exchanger 6 h to cool down the ADIP regeneration section bottoms outlet stream. In turn, the temperature of the pressurized water stream increases to between 165° F. and 175° F. (for example, about 171° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the eighth heat exchanger 6 h is between 215 MM Btu/h and 225 MM Btu/h (for example, about 219 MM Btu/h).

FIG. 1H is a schematic diagram of a natural gasoline de-colorizing section waste heat recovery system in the NGL fractionation plant. A ninth heat exchanger 6 i is located in the natural gasoline de-colorizing section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the ninth heat exchanger 6 i to cool down the natural gas de-colorizing section pre-flash drum overhead outlet stream. In turn, the temperature of the pressurized water stream increases to between 205° F. and 215° F. (for example, about 211° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the ninth heat exchanger 6 i is between 100 MM Btu/h and 110 MM Btu/h (for example, about 107 MM Btu/h).

A tenth heat exchanger 6 j is located in the natural gasoline de-colorizing section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the tenth heat exchanger 6 j to cool down the natural gas de-colorizer overhead outlet stream. In turn, the temperature of the pressurized water stream increases to between 225° F. and 235° F. (for example, about 229° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the tenth heat exchanger 6 j is between 50 MM Btu/h and 55 MM Btu/h (for example, about 53 MM Btu/h).

FIG. 1I is a schematic diagram of a propane tank vapor recovery section waste heat recovery system in the NGL fractionation plant. An eleventh heat exchanger 6 k is located in the propane tank vapor section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the eleventh heat exchanger 6 k to cool down the propane vapor recovery compressor outlet stream. In turn, the temperature of the pressurized water stream increases to between 260° F. and 270° F. (for example, about 263° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the eleventh heat exchanger 6 k is between 25 MM Btu/h and 35 MM Btu/h (for example, about 29 MM Btu/h).

FIG. 1J is a schematic diagram of a propane product refrigeration section waste heat recovery system in the NGL fractionation plant. A twelfth heat exchanger 6 l is located in the propane product refrigeration section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the twelfth heat exchanger 6 l to cool down the propane refrigeration compressor outlet stream. In turn, the temperature of the pressurized water stream increases to between 185° F. and 195° F. (for example, about 192° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the twelfth heat exchanger 6 l is between 75 MM Btu/h and 85 MM Btu/h (for example, about 81 MM Btu/h).

FIG. 1K is a schematic diagram of a propane product sub-cooling section waste heat recovery system in the NGL fractionation plant. A thirteenth heat exchanger 6 m is located in the propane product sub-cooling section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the thirteenth heat exchanger 6 m to cool down the propane main compressor outlet stream. In turn, the temperature of the pressurized water stream increases to between 235° F. and 245° F. (for example, about 237° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the thirteenth heat exchanger 6 m is between 60 MM Btu/h and 70 MM Btu/h (for example, about 65 MM Btu/h).

FIG. 1L is a schematic diagram of a butane product refrigeration waste heat recovery system in the NGL fractionation plant. A fourteenth heat exchanger 6 n is located in the butane product refrigeration section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the fourteenth heat exchanger 6 n to cool down the butane refrigeration compressor outlet stream. In turn, the temperature of the pressurized water stream increases to between 140° F. and 150° F. (for example, about 147° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the fourteenth heat exchanger 6 n is between 45 MM Btu/h and 55 MM Btu/h (for example, about 49 MM Btu/h).

FIG. 1M is a schematic diagram of an ethane production section waste heat recovery system in the NGL fractionation plant. A fifteenth heat exchanger 6 o is located in the ethane production section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the fifteenth heat exchanger 6 o to cool down the ethane dryer outlet stream during the generation mode. In turn, the temperature of the pressurized water stream increases to between 405° F. and 415° F. (for example, about 410° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the fifteenth heat exchanger 6 o is between 20 MM Btu/h and 30 MM Btu/h (for example, about 22 MM Btu/h).

FIG. 1N is a schematic diagram of a natural gasoline vapor pressure control section waste heat recovery system in the NGL fractionation plant. A sixteenth heat exchanger 6 p is located in the natural gasoline vapor pressure control section of the NGL fractionation plant. The pressurized water stream flows from the storage tank 605 to the sixteenth heat exchanger 6 p to cool down the RVP control column overhead outlet stream. In turn, the temperature of the pressurized water stream increases to between 205° F. and 215° F. (for example, about 211° F.). The heated pressurized water stream flows to the collection header to join other pressurized water streams to flow to the MED system 609. The total thermal duty of the sixteenth heat exchanger 6 p is between 30 MM Btu/h and 40 MM Btu/h (for example, about 36 MM Btu/h).

FIG. 1O is a schematic diagram of an integrated, customized, mono-refrigerant dual vapor compressor-ejector cycle 611. The mono-refrigerant used in the cycle is propane liquid at two identified, operating pressures to serve the mechanical compression refrigeration cycle, and the ejector refrigeration cycle for the compressor sub-cooling to reduce the main refrigeration cycle for the NGL fractionation plant by between 10 MW and 20 MW (for example, about 14.3 MW), which represents about 30% of the refrigeration package power consumption. Implementations of the cycle shown in FIG. 1O use high pressure liquid propane to directly recover the waste heat in the NGL fractionation plant de-ethanizer overhead stream refrigeration compressors. To do so, the high pressure liquid propane is flowed through a seventeenth heat exchanger 6 q having a thermal duty between 400 MM Btu/h and 410 MM Btu/h (for example, about 403 MM Btu/h). The high pressure liquid propane at a pressure of between 15 bar and 25 bar (for example, 20 bar) and a temperature of between 80° F. and 90° F. (for example, 88° F.) is fully vaporized in the seventeenth heat exchanger 6 q. The vapor propane is directed to work as a motive stream in a vapor ejector.

In the compressor-ejector cycle, the first liquid propane stream flows from a high pressure feed drum at a pressure of between 20 bar and 30 bar (for example, 24 bar) located after the refrigeration compressor after-cooler to an eighteenth heat exchanger 6 r that has a thermal load of between 90 MM Btu/h and 100 MM Btu/h (for example, 98 MM Btu/h). This sub-cooler uses another stream of liquid propane from the ejector cycle as shown in FIG. 1O. The second liquid propane stream is at a pressure of between five bar and 15 bar (for example, 10.8 bar). A small portion of this second propane liquid stream (for example about 20%) is throttled in a throttling valve to a pressure between five bar and 10 bar (for example, 6.25 bar) to generate chilling capacity at a temperature between 45° F. and 55° F. (for example, about 49° F.) for a thermal load of between 90 MM Btu/h and 100 MM Btu/h (for example, 98 MM Btu/h). The chilling capacity is sufficient to satisfy the needs of the NGL fractionation plant de-ethanizer overhead stream propane refrigeration compressor stream sub-cooling. The remainder of the second propane stream is pumped to a higher pressure of between 15 bar and 25 bar (for example, about 20 bar).

The two propane vapor streams—the first out of the sub-cooler at a temperature of between 50° F. and 60° F. (for example, 55° F.) and a pressure between 3 bar and 10 bar (for example, 5.9 bar), and the second out of the seventeenth heat exchanger at a temperature between 125° F. and 135° F. (for example, 131° F. and a pressure between 15 bar and 25 bar (for example, 19.3 bar)—are mixed in a customized ejector to get a vapor stream at the desired pressure for condensation using cooling water at a temperature between 70° F. and 80° F. (for example, 77° F.). The propane stream out of the ejector at a pressure between five bar and 15 bar (for example, 11 far) and a temperature between 95° F. and 105° F. (for example, 99° F.) is condensed and flowed to the ejector cycle for the compressor stream sub-cooling using the compressors low grade waste heat directly.

FIG. 1P is a schematic diagram representing an example of a modified MED system 609 configured to produce potable water using the pressurized water heated by the heat exchanger network 607. The modified MED system represented by FIG. 1P can be implemented to produce about 114,000 m³/day of potable water. The system includes multiple trains, for example, three trains, in series. The trains can be independent from each other. Each train, in turn, can include multiple effects having the same design. An effect includes a heat exchanger that uses the water heated by the heat exchanger network 607 to distill the brackish water into fresh, potable water and brine. The effects can be coupled in parallel. In some implementations, the hot waste stream heated by the heat exchanger network 607 starts at between 65° C. and 75° see (for example, about 70° C.) when flowed into the MED system and ends at between 45° C. 55° C. (for example, 49° C.) when flowed out of the MED system. The temperature of brine from which the potable water is generated is between 55° C. and 65° C. (for example, about 58° C.).

In the schematic diagram shown in FIG. 1P, the MED system 609 includes three trains. The first train 620 a can include six effects connected in series. The second train 620 b can include four effects connected in series. The third train 620C can include two effects connected in series. The number of trains and the number of effects in this implementation are examples. The MED system 609 can have fewer or more trains, with each phase having fewer or more effects. The arrangement shown in FIG. 1P represents a best match between the heat duty load and reasonable temperature drop between effects that renders best water production from the available waste heat.

The MED system feed water is distributed onto the heat exchanger of the first effect in all of the trains of the system 609. The high pressure water stream, heated by the heat exchanger network 607, flows through the heat exchanger and releases its energy to the distributed feed water to evaporate a portion of the feed water. The produced vapor then condenses in the heat exchanger of the second effect to evaporate more water in that effect. The brine from the first effect is then purged. At the second effect, the evaporated feed water goes on to power the third effect with the resulting brine being drained from the bottom of the effect. This process continues to the last effect within each train with the corresponding produced vapor entering the condenser section to be condensed by the incoming saline water acting as a coolant. Part of the pre-heated saline water is then sent to the various effects as feed water. The saline water temperature can be between 25° C. and 35° C. (for example, about 28° C.), and the feed water temperature can be between 30° C. and 40° C. (for example, about 35° C.). The temperature drop from one effect to the next can be between 1° C. and 5° C. (for example, 3° C.).

In some implementations, a steam booster unit is included in the MED system to better exploit the waste heat stream to increase the fresh water yield. The steam booster unit includes an evaporator powered by the outgoing waste heat source of the MED system. The vapor generated from the steam booster unit is introduced into a suitable effect of the MED system. The inclusion of the steam booster unit in the MED system can increase the production rate to the extent allowed by the temperature drop across the steam booster unit.

In some implementations, one or more flashing chambers can be included in the MED system to improve the efficiency of the MED system, to extract more energy from the waste heat, and to utilize the extracted energy to generate stream, thereby increasing fresh water production. In such implementations, the outlet source from the MED system goes on to heat the feed water via a liquid-liquid heat exchanger, which is slightly heated by the outlet brine stream from the last flashing chamber. The heated feed water goes through a series of flashing chambers. The vapor generated from each stage of the flashing is then injected into an effect of the MED system for further boosting.

By identifying a best match between the waste heat load temperature profile and the number of effects used in each train, the quantity of water that can be generated using the MED system is optimized.

Thus, particular implementations of the subject matter have been described. Other implementations are within the scope of the following claims. 

The invention claimed is:
 1. A method of recovering heat from a Natural Gas Liquid (NGL) fractionation plant for production of potable water, the method comprising: heating a buffer fluid via a heat exchanger in a NGL fractionation plant to transfer heat from the NGL fractionation plant to the buffer fluid, the NGL fractionation plant comprising a dehydrator column and a distillation column; and producing potable water with heat from the buffer fluid in a multi-effect distillation (MED) system comprising train distillation effects.
 2. The method of claim 1, comprising: storing the buffer fluid in a storage tank; flowing the buffer fluid from the storage tank to the heat exchanger; and flowing the buffer fluid from the MED system to the storage tank.
 3. The method of claim 1, wherein producing potable water with heat from the buffer fluid comprises heating brackish water with heat from the buffer fluid, wherein the MED system is a modified MED system, and wherein the buffer fluid comprises water or oil.
 4. The method of claim 1, wherein the NGL fractionation plant comprises a natural gas de-colorizing section comprising the distillation column as a natural gas de-colorizer distillation column, and wherein heating the buffer fluid comprises heating the buffer fluid via the heat exchanger with heat from a pre-flash drum overhead outlet stream in the natural gas de-colorizing section.
 5. The method of claim 1, wherein the NGL fractionation plant comprises an ethane production section comprising an ethane dryer vessel, and wherein heating the buffer fluid comprises heating the buffer fluid via the heat exchanger with heat from an outlet stream of the ethane dryer vessel.
 6. The method of claim 1, comprising generating cooling capacity for the NGL fractionation plant by evaporating propane in an evaporator heat exchanger in a mechanical compression refrigeration cycle comprising mono-refrigerant dual vapor compression-ejector cycles comprising the evaporator heat exchanger, a propane compressor, and an ejector.
 7. A method of recovering heat from a Natural Gas Liquid (NGL) fractionation plant for production of potable water, the method comprising: heating a buffer fluid via a heat exchanger in a NGL fractionation plant to transfer heat from the NGL fractionation plant to the buffer fluid, the NGL fractionation plant comprising a dehydrator column and a distillation column; and heating brackish water with the buffer fluid discharged from the heat exchanger for production of potable water from the brackish water via a multi-effect-distillation (MED) system comprising train distillation effects.
 8. The method of claim 7, comprising producing the potable water from the brackish water via the train distillation effects, wherein producing the potable water comprises generating brine, wherein the MED system is a modified MED system, and wherein the buffer fluid comprises water or oil.
 9. The method of claim 7, wherein the NGL fractionation plant comprises a propane de-hydration section comprising the dehydrator column as a propane dehydrator column, and wherein heating the buffer fluid comprises heating the buffer fluid via the heat exchanger with heat carried by a propane de-hydration outlet stream from the propane de-hydration section.
 10. The method of claim 7, wherein the NGL fractionation plant comprises a butane de-hydrator section comprising the dehydrator column as a butane dehydrator column, and wherein heating the buffer fluid comprises heating the buffer fluid via the heat exchanger with heat carried by a butane de-hydrator outlet stream.
 11. The method of claim 7, wherein the NGL fractionation plant comprises: another distillation column comprising a de-ethanizer distillation column; and a closed-loop propane refrigeration system comprising a mono-refrigerant dual vapor compression-ejector cycles comprising a chiller heat exchanger, a propane compressor, and an evaporator heat exchanger, wherein the method comprises: discharging overhead gas comprising ethane gas from the de-ethanizer distillation column to the chiller heat exchanger; condensing ethane gas in the chiller heat exchanger with propane as refrigerant in the closed-loop propane refrigeration system; compressing, via the propane compressor, the propane discharged from the chiller heat exchanger; and evaporating a second propane in the evaporator heat exchanger with heat transferred from the propane discharged from the propane compressor.
 12. A method of recovering heat from a Natural Gas Liquid (NGL) fractionation plant for production of potable water, the method comprising: heating a buffer fluid via a heat exchanger in a NGL fractionation plant to transfer heat from the NGL fractionation plant to the buffer fluid, the NGL fractionation plant comprising a dehydrator column, a distillation column, and a compressor; heating brackish water with the buffer fluid discharged from the heat exchanger; feeding the brackish water heated by buffer fluid to a multi-effect distillation (MED) system comprising train distillation effects; and generating potable water from the brackish water via the train distillation effects.
 13. The method of claim 12, wherein the NGL fractionation plant comprises a de-propanizer section comprising the distillation column as a de-propanizer distillation column, and wherein heating the buffer fluid comprises heating the buffer fluid via the heat exchanger with heat carried by an overhead outlet stream from the de-propanizer distillation column.
 14. The method of claim 12, wherein the NGL fractionation plant comprises a de-butanizer section comprising the distillation column as a de-butanizer distillation column, and wherein heating the buffer fluid comprises heating the buffer fluid via the heat exchanger with heat carried by an overhead outlet stream from the de-butanizer distillation column or with heat carried by a bottoms outlet stream from the de-butanizer distillation column, or a combination thereof with the heat exchanger comprising two respective heat exchangers and with the buffer fluid comprising two respective buffer fluids.
 15. The method of claim 12, wherein the NGL fractionation plant comprises a de-pentanizer section comprising the distillation column as a de-pentanizer distillation column, and wherein heating the buffer fluid comprises heating the buffer fluid via the heat exchanger with heat carried by an overhead outlet stream from the de-pentanizer distillation column.
 16. The method of claim 12, wherein the NGL fractionation plant comprises an Amine-Di-Iso-Propanol (ADIP) regeneration section comprising the distillation column as an ADIP regeneration distillation column, wherein heating the buffer fluid comprises heating the buffer fluid via the heat exchanger with heat carried by an overhead outlet stream from the ADIP regeneration distillation column or with heat carried by a bottoms stream from the ADIP regeneration distillation column, or a combination thereof with the heat exchanger comprising two respective heat exchangers and the buffer fluid comprising two respective buffer fluids.
 17. The method of claim 12, wherein the NGL fractionation plant comprises a natural gas de-colorizing section comprising the distillation column as a natural gas de-colorizer distillation column, wherein heating the buffer fluid comprises heating the buffer fluid via the heat exchanger with heat carried by an overhead stream from the natural gas de-colorizer distillation column or with heat carried by a bottoms stream from the natural gas de-colorizer distillation column, or a combination thereof with the heat exchanger comprising two respective heat exchangers and the buffer fluid comprising two respective buffer fluids.
 18. The method of claim 12, wherein the NGL fractionation plant comprises a Reid Vapor Pressure (RVP) control section comprising the distillation column as a RVP distillation column, and wherein heating the buffer fluid comprises heating the buffer fluid via the heat exchanger with heat carried by an overhead outlet stream from the RVP distillation column.
 19. A method of recovering heat from a Natural Gas Liquid (NGL) fractionation plant for production of potable water, the method comprising: heating a buffer fluid via a heat exchanger with heat from an outlet stream discharged from a compressor in the NGL fractionation plant, wherein the buffer fluid comprises water or oil; heating brackish water with the buffer fluid discharged from the heat exchanger; feeding the brackish water heated by the buffer fluid to a multi-effect distillation (MED) system comprising train distillation effects; and separating, via the MED system, the brackish water into potable water and brine.
 20. The method of claim 19, wherein the compressor is a propane vapor recovery compressor in a propane vapor recovery section in the NGL fractionation plant, the propane vapor recovery section further comprising a propane condenser heat-exchanger.
 21. The method of claim 19, wherein the compressor is a propane refrigeration compressor in a propane product refrigeration section in the NGL fractionation plant, or wherein the compressor is a butane refrigeration compressor in a butane product refrigeration section in the NGL fractionation plant, or a combination thereof with the heat exchanger comprising two respective heat exchangers and the buffer fluid comprising two respective buffer fluids.
 22. The method of claim 19, wherein the compressor is a propane main compressor in a propane product sub-cooling section in the NGL fractionation plant, the propane product sub-cooling section further comprising a propane condenser heat-exchanger.
 23. A method of recovering heat from a Natural Gas Liquid (NGL) fractionation plant, comprising: heating a buffer fluid via a heat exchanger with heat from a discharge stream of a distillation column in the NGL fractionation plant, wherein the distillation column comprises a de-propanizer distillation column, a de-butanizer distillation column, a de-pentanizer distillation column, an Amine-Di-Iso-Propanol (ADIP) regeneration distillation column, a natural gas de-colorizer distillation column, or a Reid Vapor Pressure (RVP) distillation column; heating brackish water with the buffer fluid discharged from the heat exchanger; feeding the brackish water as heated to a multi-effect distillation (MED) system comprising train distillation effects; generating potable water from the brackish water via the train distillation effects; and generating brine from the brackish water via the train distillation effects.
 24. The method of claim 23, wherein the distillation column comprises the de-propanizer distillation column, and wherein the discharge stream comprises an overhead outlet stream from the de-propanizer distillation column.
 25. The method of claim 23, wherein the distillation column comprises the de-butanizer distillation column, and wherein the discharge stream comprises an overhead outlet stream from the de-propanizer distillation column or a bottoms outlet stream from the de-butanizer distillation column.
 26. The method of claim 23, wherein the distillation column comprises the de-pentanizer distillation column, and wherein the discharge stream comprises an overhead outlet stream from the de-pentanizer distillation column.
 27. The method of claim 23, wherein the distillation column comprises the ADIP regeneration distillation column, and wherein the discharge stream comprises an overhead outlet stream from the ADIP regeneration distillation column or a bottoms stream from the ADIP regeneration distillation column.
 28. The method of claim 23, wherein the distillation column comprises the natural gas de-colorizer distillation column, and wherein the discharge stream comprises an overhead stream from the natural gas de-colorizer distillation column or a bottoms stream from the natural gas de-colorizer distillation column.
 29. The method of claim 23, wherein the distillation column comprises the RVP distillation column, and wherein the discharge stream comprises an overhead outlet stream from the RVP distillation column distillation column. 